1. Field of the Invention
This invention relates to interoffice DC loop signaling arrangements for communication switching systems and more particularly to solutions of false reseizure problems attributable to facility-side midpoint capacitors associated with repeat coils utilized in certain loop-signaling interoffice telecommunication trunk circuits.
2. Description of the Prior Art
Known interoffice trunks providing for metallic voice circuits between communication switching systems commonly employ DC supervisory and address signaling methods. Such methods utilize open and closed DC current loops, as well as normal and reverse battery potential across the two-wire facility, in conjunction with trunk circuit supervisory relays and/or reverse battery detector circuits to convey signaling information between trunk circuits terminating the end points of the interoffice trunk facility. Such trunk circuits are commonly known in the art as loop-signaling or loop-dial trunk circuits.
Additionally, as is well-known in the art, many of such trunk circuits employ impedance matching transformers commonly known as repeat coils. Repeat coils are characterized as having a pair of windings on both the primary and secondary of the impedance matching transformer, each pair interconnected by so-called midpoint capacitors. Rather than using the terms primary and secondary with reference to the transformer windings, the terms office-side and facility-side are used when discussing repeat coils. These designations flow naturally from considering that which happens to be connected to a particular side of a repeat coil under discussion -- i.e. the telephone switching office, or "office-side," or the interoffice transmission trunk facility, or "facility-side."
Such prior art loop-signaling incoming trunk circuits with repeat coil coupling, may encounter false reseizure problems at the termination of the talk state due to charge build-up on the facility-side repeat coil midpoint capacitors in the outgoing trunk circuits. Under such a condition, the facility-side midpoint capacitor in the outgoing trunk circuit may discharge during the newly-initiated idle trunk state in a manner resulting in the incorrect operation of a supervisory relay in the incoming trunk circuit at the distant office. Indeed, such a false reseizure, under certain conditions, may become reiterative, resulting in a condition known as "flipping," wherein the distant office incoming trunk circuit is repetitively falsely seized over a prolonged time interval. Such false incoming trunk seizures are accompanied by false requests for common control circuit elements such as incoming registers, thus degrading service efficiency at the distant office associated with the incoming trunk circuit.